Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used in refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable/frozen product storage area in commercial establishments.
Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable/frozen items by truck, rail, ship or intermodally. Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to more stringent operating conditions due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature. The desired temperature at which the cargo needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved. The refrigerant vapor compression system must not only have sufficient capacity to rapidly pull down the temperature of product loaded into the cargo space at ambient temperature, but also operate efficiently at low load when maintaining a stable product temperature during transport. Additionally, transport refrigerant vapor compression systems are subject to vibration and movements not experienced by stationary refrigerant vapor compression systems.
Traditionally, most of these refrigerant vapor compression systems operate at subcritical refrigerant pressures and typically include a compressor, a condenser, and an evaporator, and expansion device, commonly an expansion valve, disposed upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit, arranged in accord with known refrigerant vapor compression cycles, and operated in the subcritical pressure range for the particular refrigerant in use. Refrigerant vapor compression systems operating in the subcritical range are commonly charged with fluorocarbon refrigerants such as, but not limited to, hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly hydrofluorocarbons (HFCs), such as R134a, R410A, R404A and R407C.
In today's market, greater interest is being shown in “natural” refrigerants, such as carbon dioxide, for use in air conditioning and transport refrigeration systems instead of HFC refrigerants. However, because carbon dioxide has a low critical temperature, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime. In refrigerant vapor compression systems operating in a subcritical cycle, both the condenser and the evaporator heat exchangers operate at refrigerant temperatures and pressures below the refrigerant's critical point. However, in refrigerant vapor compression systems operating in a transcritical cycle, the heat rejection heat exchanger, which is a gas cooler rather than a condenser, operates at a refrigerant temperature and pressure in excess of the refrigerant's critical point, while the evaporator operates at a refrigerant temperature and pressure in the subcritical range. Thus, for a refrigerant vapor compression system operating in a transcritical cycle, the difference between the refrigerant pressure within the gas cooler and refrigerant pressure within the evaporator is characteristically substantially greater than the difference between the refrigerant pressure within the condenser and the refrigerant pressure within the evaporator for a refrigerant vapor compression system operating in a subcritical cycle.
It is also common practice to incorporate an economizer into the refrigerant circuit for increasing the capacity of the refrigerant vapor compression system. For example, in some systems, a refrigerant-to-refrigerant heat exchanger is incorporated into the refrigerant circuit as an economizer. A first portion of the refrigerant leaving the condenser passes through a first pass of the heat exchanger in heat exchange with a second portion of the refrigerant passing through the second pass of the heat exchanger. The second portion of the refrigerant typically constitutes a portion of the refrigerant leaving the condenser that is diverted through an expansion device wherein this portion of the refrigerant is expanded to a lower pressure and a lower temperature vapor or vapor/liquid mixture refrigerant before this second portion of refrigerant is passed through the second pass of the economizer refrigerant-to-refrigerant heat exchanger. Having traversed the second pass of the economizer heat exchanger, the second portion of the refrigerant is thence directed into an intermediate pressure change of the compression process. The refrigerant in the primary refrigerant circuit passes through the first pass of the refrigerant-to-refrigerant economizer heat exchanger and is thus further cooled before it traverses the system's main expansion device prior to entering the evaporator. U.S. Pat. No. 6,058,729 discloses a subcritical refrigerant vapor compression system for a transport refrigeration unit incorporating a refrigerant-to-refrigerant heat exchanger into the refrigerant circuit as an economizer.
In some systems, a flash tank economizer is incorporated into the refrigerant circuit between the condenser and the evaporator. In such case, the refrigerant leaving the condenser is expanded through an expansion device, such as a thermostatic expansion valve or an electronic expansion valve, prior to entering the flash tank wherein the expanded refrigerant separates into a liquid refrigerant component and a vapor refrigerant component. The vapor component of the refrigerant is thence directed from the flash tank into an intermediate pressure stage of the compression process. The liquid component of the refrigerant is directed from the flash tank through the system's main expansion valve prior to entering the evaporator. U.S. Pat. No. 5,174,123 discloses a subcritical vapor compression system incorporating a flash tank economizer in the refrigerant circuit between the condenser and the evaporator.
In conventional subcritical refrigerant vapor compression systems, the expansion of the refrigerant passing from the condenser to the evaporator is typically a single step process wherein the refrigerant passing from the condenser to the evaporator through a single expansion device, commonly either a thermostatic expansion valve or an electronic expansion valve or a fixed orifice device, before the refrigerant enters the evaporator. U.S. Pat. No. 6,694,750 discloses a subcritical refrigeration system that includes a first refrigerant-to-refrigerant heat exchanger economizer and a second refrigerant-to-refrigerant heat exchanger economizer disposed in series in the refrigerant circuit between the condenser and the evaporator. Refrigerant passing through the primary refrigerant circuit from the condenser to the evaporator passes in series through the first pass of the first refrigerant-to-refrigerant heat exchanger and thence through the first pass of the second refrigerant-to-refrigerant heat exchanger before traversing the primary refrigerant circuit's single evaporator expansion valve prior to entering the evaporator. A second portion of the refrigerant passing from the condenser is diverted from the primary refrigerant circuit and passed through an auxiliary expansion valve and thence through the second pass of the first refrigerant-to-refrigerant heat exchanger prior to being injected into a high pressure stage of the compression process. A third portion of the refrigerant passing from the condenser is diverted from the primary refrigerant circuit and passed through another auxiliary expansion valve and thence through the second pass of the second refrigerant-to-refrigerant heat exchanger prior to being injected into a low pressure stage of the compression process.
U.S. Pat. No. 6,385,980 discloses a transcritical refrigerant vapor compression system incorporating a flash tank economizer in the refrigerant circuit between the gas cooler and the evaporator. However, in a transcritical refrigerant vapor compression system, expanding the refrigerant from the supercritical pressure at which the gas cooler operates to the subcritical pressure at which the evaporator operates in a single step expansion process may result in lower system efficiency and over-heating of the compressor due to the large pressure differential between the gas cooler pressure and the evaporator pressure.